1. Field of the Invention
The present invention relates to an ultrasonic motor configured to cause a vibrator to generate an elliptical vibration and drive the vibrator pressed against a driven member.
2. Description of the Related Art
Ultrasonic motors are characterized by, for example, a noiseless operation, the capability of driving an object at various speed levels from a low speed to a high speed, and an output of a high torque. To make full use of these characteristics, conventionally, the ultrasonic motors have been employed as, for example, driving sources of cameras and lenses. Japanese Patent No. 4667839 discusses an ultrasonic motor including an annular driven member having a rotational shaft and a plurality of vibrators. The vibrators are disposed on the annular driven member so as to be spaced apart at predetermined intervals while being pressed against the driven member in frictional contact with the driven member. When an ultrasonic vibration is excited at each of the vibrators in frictional contact with the driven member, an elliptical motion is generated at a portion of the vibrator that contacts the driven member, and the driven member is rotationally driven around the rotational shaft. The pressing force of the vibrator against the driven member is acquired by biasing a portion near a neutral axis of the vibrator, which corresponds to a vibration node set near a center of the vibrator, with a plate spring via a holder member and a pressing member. Then, this pressing force is adjusted with use of a screw and an adjustment washer disposed near a fixed portion of the plate spring.
However, a holding mechanism of the vibrator in the ultrasonic motor discussed in above-described Japanese Patent No. 4667839 is constituted by many components and has a complex structure. Especially, a support shaft should be located near the neutral axis of the vibrator to hold the vibrator in a stably pressed state against the driven member. There are a large number of components and assembling the components is complicated. Further, this ultrasonic motor uses a large piezoelectric element in the structure, and, therefore, causes an increase in cost.